Inertial navigation of a platform is based upon the integration of specific forces and angular rates measured by inertial sensors (e.g. accelerometer, gyroscopes) by a device containing the sensors. In general, the device is positioned within the platform and commonly tethered to the platform. Such measurements from the device may be used to determine the position, velocity and attitude of the device and/or the platform.
The platform may be a motion-capable platform that may be temporarily stationary. Some of the examples of the platforms may be a person, a vehicle or a vessel of any type. The vessel may be land-based, marine or airborne.
Alignment of the inertial sensors within the platform (and with the platform's forward, transversal and vertical axis) is critical for inertial navigation. If the inertial sensors, such as accelerometers and gyroscopes are not exactly aligned with the platform, the positions and attitude calculated using the readings of the inertial sensors will not be representative of the platform. Fixing the inertial sensors within the platform is thus a requirement for navigation systems that provide high accuracy navigation solutions.
For tethered systems, one known means for ensuring optimal navigation solutions is to utilize careful manual mounting of the inertial sensors within the platform. However, notwithstanding such careful mounting, existing portable navigation devices (or navigation-capable devices) cannot achieve accurate attitude and position of the platform unless at least one of the following three conditions are known:
1) absolute attitude angles for the device and the platform;
2) absolute attitude angles for the device and the misalignment between the device and platform;
3) absolute attitude angles for the platform and the misalignment between the device and platform.
Some known portable navigation devices that are designed for in-vehicle navigation attempt to use the platform's velocity or acceleration along with at least one or more additional constraints to estimate the mounting misalignment between the device and the platform. A known method uses the acceleration measurements and the steering rate of the platform to estimate the misalignment. The three attitude angles are determined when the platform is accelerating or turning. This method also uses the physical constraint that the user must mount the device in approximately forward orientation within the platform to make the display of the device facing the user. Similarly, it is known to provide a method where the misalignment can be resolved if the platform velocity satisfies certain criteria and the device is not moved during the trajectory. This method utilizes non holonomic constraints which are based on the fact that during normal operation a platform, such as a land vehicle, will not skid or jump. Both of these methods calculate absolute heading for the platform using GNSS velocities.
The above methods, however, are not applicable for circumstances where the movement of the platform may not satisfy the required criterion, for example, where the platform is a person walking. Several attempts have been made to find methods that can resolve the common misalignment cases for such circumstances (e.g. mobile/smart phone based inertial navigation), such as for example, application of principle component analysis to determine the forward motion for specific orientation of the device in addition to the gravity axis to find the vertical axis of the device (the one measuring the biggest component of gravity).
For navigation, mobile/smart phones are becoming very popular as they come equipped with Assisted Global Positioning System (AGPS) chipsets with high sensitivity capabilities to provide absolute positions of the platform (e.g. user) even in some environments that cannot guarantee clear line of sight to satellite signals. Deep indoor or challenging outdoor navigation or localization incorporates cell tower ID or, if possible, cell towers trilateration for a position fix where AGPS solution is unavailable. Despite these two positioning methods that already come in many mobile devices, accurate indoor localization still presents a challenge and fails to satisfy the accuracy demands of today's location based services (LBS). Additionally, these methods may only provide the absolute heading of the platform without any information on the device's heading.
As WiFi hotspots are present in abundance in indoor environments, fingerprinting and signal strength mapping may be implemented to determine the platform's position and heading for the challenging navigation scenarios. This is an attractive new method to avoid the shortcomings of AGPS and cell phone based positioning solution. However, frequent access point (AP) surveys are required to keep the WiFi related information up to date for acceptable positioning accuracies of mobile devices within the network of APs. Furthermore, the current available techniques need pre-knowledge about the environments and there are no known techniques for WiFi positioning in any new or user-unknown environment. WiFi based positioning and localization also provides absolute information for the platform with no information for the device.
Many mobile devices, such as mobile phones, are equipped with Micro Electro Mechanical System (MEMS) sensors that are used predominantly for screen control and entertainment applications. These sensors have been considered unusable to date for navigation purposes due to very high noise, large random drift rates, and frequently changing orientations with respect to the carrying platform or person.
Magnetometers are also found within many mobile devices. In some cases, it has been shown that a navigation solution using accelerometers and magnetometers may be possible if the user is careful enough to keep the device in a specific orientation with respect to their body, such as when held carefully in front of the user after calibrating the magnetometer.
Some known methods to provide navigation solutions utilize the mobile device sensors and digital maps of the buildings for pure indoor navigation. Again, however, these methods require that the mobile device be kept in a specific orientation to utilize the magnetometer-based heading and the indoor maps have to be available for the buildings that the user is entering. The maps are not only used to display the positions but they may also be used to get the heading of the user and help getting the user's position.